There has been known an image forming apparatus which a developing cartridge is attachable to and detachable from. The developing cartridge stores toner as a developing agent. This type of image forming apparatus determines whether the amount of toner in the developing cartridge decreases or whether the number of printed sheets exceeds a predetermined number. If determining that the amount of toner decreases or that the number of printed sheets exceeds the predetermined number, the image forming apparatus shows information on a display thereof to notify a user to change the developing cartridge. The user who has been noticed by the information on the display changes the developing cartridge to a new cartridge.